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506                                CHEMICAL ENGINEERING
The reaction consisting in the inversion of quartz to cristobalite involves a linear expansion of 4.35, that of quartz to tridymite, 5.3 per cent. Although tridymite would thus appear to be the end product, it is not feasible under commercial conditions to carry the inversion that far. We must ordinarily be satisfied to obtain 70 per cent of cristobalite and a small amount of tridymite. In the transformation of quartz to cristobalite the density drops from 2.65 to 2.33, equivalent to a volume increase of 13.6 per cent. The change to tridymite reduces the density of quartz to 2.27, with a volume increase of 16.8 per cent. The manufacture of silica bricks consists essentially in converting the crushed quartzite, with the aid of lime, to a structure consisting essentially of about 70 per cent cristobalite, 25 of quartz, not yet inverted, and 5 per cent of tridymite. The after expansion is hence principally due to the residual quartz. From 'the standpoint of constancy in volume it would be desirable if the tridymite form predominated since the cristobalite itself is subject to a considerable volume change between 225 to 275C., from the alpha to the beta modification which has a bearing in the spalling of silica bricks. Upon refiring silica refractories and in use, the amount of tridymite increases steadily at the expense of the cristobalite and quartz until the whole mass may consist of the crystalline constituent in question.
Sandstones, as a rule, invert too slowly. Cherts are transformed rapidly even to tridymite but show poor mechanical strength owing to the general cracking of the particles.
The manufacture of silica refractories consists in the grinding of the previously crushed quartzite in a wet pan, the addition of the slaked lime and the molding of the prepared mixture by hand, in sanded steel molds. The pieces are then dried in tunnels and fired in down-draft kilns for about 9 days, carrying the temperature to the softening point of cones 16 to 18. The cooling requires from 5 to 7 days. The tunnel-car kiln is used in at least one plant, with a maximum heat treatment equivalent to cone 12 to 14.
Properties of Silica Refractories.The chemical composition of a typica material of this kind made from Pennsylvania quartzite is as follows:
PER                                                                  PER
CENT                                                              CENT
Silica.................... 96.25       Lime.....................    1.80
Alumina.................    0.88       Magnesia.................    0.14
Ferric oxide..............    0.79       Alkali.....................    0.39
The softening point of the best silica materials is about that of cone 32; it should not fall below that of cone 31.
The permanent linear expansion in the well-burnt product is not more than 2 per cent of the original length when the specimen is reheated to 1,500C. In the use of silica bricks expansion joints 0.25 in. per foot must be provided. The porosity of silica bricks is usually between 24 to 27 per cent. The specific gravity for material made from Medina quartzite is usually below 2.38 and that from Baraboo quartzite below 2.42. These values may be used in specifications to insure the proper degree of firing.
Silica bricks withstand load conditions satisfactorily even at temperatures up to 1,500C., under a pressure of 50 Ib. per square inch. The specific heat of silica refractories varies from 0.2 to 0.266, between 0C. and 1,200. The thermal conductivity, in calories per second, through 1 cm.3 per degree Centigrade temperature difference, was found to be 0.0021, for the temperature range 0 to 100C., and 0.0031, for 0 to 1,000C. Dudley calculates the mean conductivity between any two temperatures, ti and fe, from the relation: K  0.0020 -f 0.0000011 (ti + fe).1 Stockman and
i Trans. Am. Electrochera. Soc., Vol. 27, p. 285 (1915).